It is well known that SVM systems may enhance cathode ray tube picture sharpness by modulating the scanning velocity of an electron beam based on a differentiated video signal, or SVM signal, derived from the luminance component of a video display signal. Slowing the scanning velocity of the electron beam causes a greater number of electrons to land at a particular point in the display resulting in a brighter picture at that particular location on the display. In contrast, increasing the velocity of the electron beam results in fewer electrons striking the display which leads to a darker picture at that particular location. The net effect of such modulation causes variations in display intensity about edge transitions in the picture resulting in the perception of increased picture sharpness. It is desirable, however, to disable SVM operation under certain conditions, for example, when channels are being changed, computer images displayed or when on screen display (OSD) message signals are generated for display. In addition, the output stages of an SVM circuit must be controlled to prevent over dissipation (excessive temperatures) in those stages.
Various schemes have been used to accomplish these objectives. For example, SVM systems are known which include a control circuit for protecting output stage devices and a disabling circuit for disabling an SVM circuit during OSD operation. It is also known to control SVM signal amplitude in accordance with output stage current to prevent excessive dissipation in output stage devices. Such systems, however, suffer from several disadvantages. However, SVM inhibition and the prevention of over dissipation are facilitated by separate, independent systems which leads to a greater number of components and increased costs.
Thus, what is needed is an SVM control circuit that accomplishes both of these important objectives through the use of a minimum number of components. Reducing the number of parts that is needed to successfully operate an SVM system leads to lower costs.